Web Feature Services – real data from the web to your desktop
This post looks at web feature services (WFS) where real data from internet sources can be added to your desktop for analysis purposes.
In a previous post we looked at web services that could be added to a map. While these are great for map making purposes, they do have their limitations, especially if you want to do any proper analysis. Technically, those web services are referred to as Web Mapping Services (WMS). Another flavour of web services is Web Feature Services (WFS) which give you direct access to actual features from your desktop.
****NOTE****: To take advantage of WFS, you’ll need to have a new extension installed, the Data Interoperability extension (try saying that ten times…) which allows you to make the right connection. Contact myself or Paul Helleur at ITS to have this installed.
First, an overview of the process followed by some detail:
- Get a URL to the feature service
- Create a new interoperability connection
- Add data to your map.
Get a URL to the feature service
This is probably the trickiest part. Anytime a web service of any kind is made available, there’s a record of it on the server. For example, CERA has a lot of their data offered up as WMS. There’s a web site on every GIS server where you can find summaries of all the web services on offer – click on the link below or copy and paste it into your favourite browser (which I hope is NOT Internet Explorer) to see CERA’s:
This gets you to the Services Directory – a summary of all the services available in the CERA folder on the maps.cera.govt.nz server.
The “/arcgis/rest/services” bit is where all the services live, so if you know the name of a particular server (e.g. gis.lincoln.ac.nz), you could add on the “/arcgis/rest/services” to get to the Services Directory. Try it if you like.
Clicking on any one of the links will get you some more information on that service, but most of it will likely be gibberish until you become familiar with the intricacies of web mapping. Here’s the page for the CERA_TechClasses_WGS84 service:
With a bit of wherewithal, I could easily add this map service to an ArcMap map, but again, it would just be an image rather than useful data. Just above the bold faced title are a few links: “JSON | SOAP | WMS | WFS”. JSON and SOAP are internet protocols for working with data objects, WMS means we could add the map service to a map; WFS is the key here – it means that we can get at the actual data. Click on that link takes you here:
Yes, more gobbeldy gook but let’s focus on the URL (uniform resource locator) in the browser address line:
http://maps.cera.govt.nz/arcgis/services/CERA/CERA_TechClasses_WGS84/MapServer/WFSServer?request=GetCapabilities&service=WFS
This is what we’ll need to add this layer to a map – copy it.
Create a new interoperability connection
So the URL gives us the pathway to the data. Next we’ll use the interoperability extension to connect to the data – do this in ArcCatalog. In the Catalog Window look at the bottom of the list for Interoperability Connections:
Double-click “Add Interoperability Connection”. In the window that pops up, paste the URL from above into the “Dataset” window and set the “Format” window to WFS
Now click the “Parameters” button. Tick the “Use Proxy Server” box and put in these details to get through the firewall (make sure you use “lu\yourusername” instead of mine).
Then click the Contraints > Feature Types button with the ellipses:
might take a bit, but you’ll then get a window of available layers:
Tick the ones you want and click OK. Here I’ve only ticked one – these are all versions from different releases of the technical classes (how they changed over time). Click OK three times to finish.
We’ve got the connection set up – now we can add the layer to a map.
Add data to your map
In ArcMap, click Add Data and navigate to the Interoperability connection we set up – add the CERA layer to your map et voila!
Now, unlike a map service, we’ve got full access to the features and the attribute table. Plus we can use this layer in geoprocessing tools. This layer will run more slowly than a local copy depending on how well the internet is working so a next step might be to make a local copy of the layer. (Right-click on the layer name and go to “Export Data” then save it as a shapefile or feature class somewhere locally.) I’ve already done this and you can find a copy of this layer in J:\Data\Christchurch\CERA.
One limitation of the CERA data is that the WFS layers are only available in WGS84 (latitude and longitude, the coordinate system that GPS uses) whereas most analyses will be done in, say, NZTM. I’m not quite sure why this is, but another step with this layer might be to project it to NZTM so it plays nicely with other data. You could do this straight from the connection in ArcCatalog or from ArcMap.
Feature services are a powerful way of making full blown versions of data available over the internet – gone may be the days of huge zip files or waiting for that DVD in the post (though it does mean that you need someone around who can set all this up.) Happily, CERA’s not the only organisation to offer WFS. Both LINZ and Landcare offer data access this way (at data.linz.govt.nz and lris.scinfo.org.nz) , but it is slightly more complicated. I’ll cover how to work with those sites in another post.
C
